1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system, and more particularly, to a method of handling random access for a user equipment with coverage enhancement in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
A machine type communication (MTC) device which can automatically perform predefined jobs and report corresponding results to other devices, a server, a Node-B (NB) or an eNB can be used in various areas, such as security, tracking and tracing, payment, healthcare, metering, etc. Further, the MTC device preferably reports the corresponding results via a wireless link such that limitation caused by environment can be removed. However, the wireless link used by the MTC device is needed to be established, and radio resources required by the wireless link is needed to be allocated (i.e., assigned). Reuse of existing infrastructures and wireless communication systems becomes a viable choice for operations of the MTC device. Therefore, the UMTS, the LTE system and the LTE-A system, etc., developed by the 3GPP which are widely deployed are suitable for the operations of the MTC device. An MTC device is considered to be a type of UE.
Some MTC devices may be installed in the basements of residential buildings or locations shielded by foil-backed insulation, metalized windows or traditional thick-walled building construction, and these devices would experience more significant penetration losses on the radio interface than normal LTE devices. The MTC devices in the extreme coverage scenario might have characteristics such as very low data rate, greater delay tolerance and no mobility, and therefore some messages/channels may not be required. In such an extreme coverage scenario, MTC UEs work at an enhanced coverage mode in which signaling, data transmission and radio resources may be of different forms than those for the UEs in the ordinary environment.
More energy can be accumulated to improve coverage by prolonging transmission time. The existing transmission time interval (TTI) bundling and hybrid automatic repeat request (HARQ) retransmission in data channel can be helpful. Note that since the current maximum number of UL HARQ retransmissions is 28 and TTI bundling is up to 4 consecutive subframes, TTI bundling with a larger TTI bundle size may be considered and the maximum number of HARQ retransmissions may be extended to achieve better performances. Other than the TTI bundling and HARQ retransmission, repetition can be applied by repeating the same or different redundancy versions (RV) multiple times. In addition, code spreading in the time domain can also be considered to improve coverage. MTC traffic packets could be radio link control (RLC) transmission segmented into smaller packets; very low rate coding, lower modulation order (e.g. BPSK) and shorter length cyclic redundancy check (CRC) may also be used. New decoding techniques (e.g. correlation or reduced search space decoding) can be used to improve coverage by taking into account the characteristics of the particular channels (e.g., channel periodicity, rate of parameter changes, channel structure, limited content, etc.) and the relaxed performance requirements (e.g. delay tolerance).
When a UE initiates a random access procedure to get uplink synchronization with an eNB, the UE transmits a random access preamble. Once the random access preamble is transmitted, the UE shall monitor the physical downlink control channel (PDCCH) for random access response(s) identified by the random access radio network temporary identifier (RA-RNTI) defined below, in a random access response window which starts at the subframe that contains the end of the preamble transmission plus 3 subframes and has a length equal to ra-ResponseWindowSize, (e.g., 10 subframes). The RA-RNTI associated with the physical random access channel (PRACH) in which the random access preamble is transmitted is computed as:RA-RNTI=1+t_id+10×f_id;where t_id is the index of the first subframe of the specified PRACH (0≦t_id<10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0≦f_id<6). The UE may stop monitoring for random access response(s) after successful reception of a random access response containing random access preamble identifiers that match the transmitted random access preamble.
When a UE (e.g., an MTC device) initiates a random access procedure in the enhanced coverage mode, the UE performs repetitions of a random access preamble transmission to an eNB. However, the UE starts to monitor the PDCCH for random access response(s) identified by the RA-RNTI in the random access response window which starts at the subframe that contains the end of the preamble transmission plus 3 subframes and has a length equal to ra-ResponseWindowSize (e.g., 10 subframes). If the UE cannot receive the random access response containing a random access preamble identifier corresponding to the transmitted random access preamble in the random access response window, the UE determines that the random access response reception is not successful. Due to the unsuccessful random access response reception, in one case, the UE may consider there is a problem and trigger a radio resource control (RRC) connection re-establishment procedure. In another case, the UE may delay subsequent random access preamble transmission(s) by a backoff time. However, the UE in the enhanced coverage mode unnecessarily meets one of the two cases but actually the UE has not finished repetitions of the random access preamble transmission. Therefore, the UE in the enhanced coverage mode has even worse performance in the random access procedure due to interruption of the random access procedure.
Thus, there is a need for improvement over the prior art.